The present invention relates generally to active optoelectronic devices, and more specifically to such devices based upon bistable vertical-cavity lasers.
A bistable optical device is one which exhibits some optical property (e.g., transmissivity, reflectivity, polarization activity, emission intensity, emission polarization, etc.) which, for a range of input parameters, takes on one of two distinct parameters. Such a device is said to be in one state or the other, depending on the value of the bistable optical property. A state need not represent a unique value of the parameters describing the bistable optical property, but rather can describe a typical range. Hence, a device which exhibits transmissivity of 90-95%, or of 5-15%, would have two distinct states.
Bistable devices often show some degree of hysteresis in their switching behavior. That is, the state of such a device does not always depend on the current inputs and conditions, but can also be history-dependent. An example from electronic bistable devices would be a digital switch, such as a digital flip-flop.
A flip flop initially in the low state remains in the low state until a sufficiently large control voltage V+ is input, whereupon the flip flop switches to the high state. Conversely, a flip flop initially in the high state remains in the high state until a sufficiently low control voltage Vxe2x88x92 is input, whereupon the flip flop switches to the low state.
Typically, however, V+ is larger than Vxe2x88x92, so that there is an intermediate range of control voltages for which the device state depends on the history. If the device was in the low state before applying an intermediate control voltage, it remains in the low state, whereas if the device was in the high state before applying an intermediate control voltage, it remains in the high state. The state of the device is thus history-dependent for some range of the control voltage.
Hysteresis can complicate a number of applications, but contributes greatly to the use of bistable devices as switches. Without hysteresis (e.g., when V+=Vxe2x88x92), the state of a bistable switch near the switching threshold depends sensitively upon the exact value of the input, and such a switch is therefore very sensitive to noise and imperfect inputs.
However, a bistable device without hysteresis can enable another set of applications. Obvious possibilities include pulse shapers, limiters, and thresholding elements.
Additionally, in most real devices, the switch from one state to another in a bistable device without hysteresis is not infinitely sharp. As a result, there can be a region of input parameters within which a small change of those parameters results in a large change in the bistable output property. Such a device can produce large differential gain (or other response), and hence act as an amplifier. Bistable optical devices have a plethora of potential applications, the best known of which lie in the regions of optical switching, optical data transmission, and optical computing. Most current bistable optical devices depend on the interaction of two characteristics, nonlinearity and feedback.
In purely optical bistable devices, the nonlinearity is typically dispersive or dissipative, and results from the use of optical materials which have intrinsically nonlinear optical response. Feedback is provided in any of a number of ways common to laser applications.
In hybrid bistable optical devices, the nonlinearity is typically provided by applying an external field to a nonlinear optical material. This results in a macroscopic change in refractive index, polarization shift, or some other suitable parameter, which change then alters the state of the device. Feedback is provided electrically, e.g., by monitoring the optical output of the device, and in response changing the intensity of the external field. Such devices have the advantage that they can be used effectively with optical throughput of much smaller intensity than is typically required of intrinsic bistable optical devices, in which the throughput must have sufficient intensity to drive the optical nonlinearity.
Lasers commonly exhibit some degree of optical bistability regarding their lasing threshold. In semiconductor lasers, for example, a larger excitation current is required to initiate lasing than is required to maintain lasing. In principle this bistability is useful for applications involving optical signal processing, high density optical memory, and optical interconnections, as well as hybrid optoelectronic computation.
Bistable vertical-cavity lasers are of special interest for such applications, because of the high 1-D and 2-D device densities which can be achieved using straightforward integration techniques. However, the size of the hysteretic region in conventional vertical-cavity surface-emitting lasers (VCSELs) is quite small, thus limiting the use of such devices as bistable switching elements.
There is therefor a clear need for a new type of vertical-cavity laser device which exhibits a large and stable hysteretic region.
Conventional vertical-cavity surface emitting lasers (VCSELs) generally comprise a slab of active laser gain medium sandwiched between a pair of dielectric mirrors, these substructures being monolithically integrated on the surface of a semiconductor substrate. Such devices are usually lattice-matched or strained-layer structures, made of single-crystal direct-gap semiconductor materials. The most common materials used for construction are gallium arsenide based semiconductor alloys. VCSELs are commonly used to optically transfer information from one subsystem to another, either via free-space coupling or by coupling to a fiber optic.
A class of bistable semiconductor vertical-cavity laser devices has been invented. These devices comprise multiple vertically coupled resonators, each such resonator containing regions of electrically-pumped laser gain media. Each region of laser gain media can be independently injected with current, resulting in an operating parameter space with distinct regions of optical behavior. These devices exhibit large regions of hysteretic behavior, thereby enabling their use in optical switching and data encoding applications.